The study is based on the epidemic of 50,000 cases of viral hepatitis in the United States Army in 1942, traced to yellow fever vaccine prepared by the Rockefeller Foundation and contaminated with a virus of hepatitis, now thought to have been the hepatitis B virus (HBV). A serologic survey to identify the virus with certainty will be performed by the Veterans Administration and the Liver Diseases Section of the National Institute of Arthritis, Diabetes, and Digestive and Kidney Diseases on about 600 men, 200 who suffered from acute hepatitis during the 1942 epidemic, 200 who received vaccine from one of the seven contaminated lots but were not clinically ill, and 200 who did not receive the Rockefeller vaccine. Two epidemiologic studies will be performed with the Medical Follow-up Agency of the National Research Council: 1) a mortality study of three cohorts of 20,000 men each defined as in the serologic survey, with primary liver cancer the chief endpoint; and 2) a case-control study of an estimated 1,400 WWII Army Veterans discharged from VA hospitals for primary liver cancer and 2,800 matched controls, the comparison to be based on immunization history with attention to the lot number of the yellow fever vaccine.